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Published online by Cambridge University Press: 04 July 2016
Exact solutions for certain types of non-linear systems where the non-linearity is symmetrical, that is to say, gravitational effects have been ignored, and damping is absent, are given by Den Hartog and Mikina. The algebra involved in these solutions is somewhat cumbersome and inconvenient to apply. A numerical method is given by Stormer but the computation is tedious without the use of a digital computer. The method employed by Lourie and Tchekmarev is commendable by virtue of its simplicity.